1. Field of the Invention
The present invention generally relates to fiber optic rotation sensors or gyroscopes. More particularly, this invention relates to an improved fiber optic gyroscope adapted to efficient matching of effective coil length to phase modulation frequency in an open-loop rotation sensing system.
2. Description of Related Art
Fiber optic rotation sensors or gyros, as they are commonly called, are increasingly being used for detection of rotation, particularly in navigation systems where accurate and reliable sensing of inertial rotation is highly critical, such as those used in aircraft, spacecraft and related defense applications.
In comparison with rotation sensing systems using mechanical gyroscopes, fiber optic gyros offer several distinct advantages including the absence of moving parts, warm-up time and g-sensitivity. In particular, the liberation from the unavoidable problems associated with moving parts, and the extreme cost reduction and potential for high reliability realized thereby, makes fiber optic gyros highly desirable for use in inertial navigation systems.
In a typical fiber optic gyro light from a laser or some other suitable light source is divided into two separate beams by means of some form of a beam splitter and then coupled into the two ends of a multiturn coil of optical fiber, typically of the single-mode type. Light emerging from the two fiber ends is combined by the beam splitter and detected by a photodetector.
Rotation sensing is typically accomplished by detection of a rotationally induced phase shift, commonly referred to as the "Sagnac Phase Shift", between the light beams propagating in opposite directions around the closed loop formed by the coil of optical fiber. The detected signal corresponding to the phase difference between the opposing beams is typically subjected to some form of phase modulation and the photodetector converts the modulation to an electric signal which is indicative of the degree of rotation of the fiber coil and is electronically processed to provide a direct indication thereof.
In fiber optic gyros of the above type, the sensitivity of a gyro having a fixed coil diameter is directly proportional to the distance travelled by the counter-propagating beams within the fiber coil. Thus, sensitivity may be enhanced by increasing the length of the fiber by winding more turns on the coil. Further, since the gyro modulation frequency is inversely proportional to fiber length, it is desirable to maintain the fiber length at levels which realize a convenient modulation frequency.
The finite signal attenuation levels in optical fiber generally restricts the maximum length of fiber which can be used for accurate signal detection and processing. However, a more important consideration in operating fiber optic gyros is maintaining a threshold degree of linearity between the gyro output and the rotation being sensed. Linearity of the gyro output is proportional to the degree of phase shift realized for a given rotation rate which, in turn, is proportional to the coil diameter and fiber length. Because of inherent constraints on the minimum bending radius of optical fibers, the only remaining practical approach to maximizing output linearity is to decrease the coil length which, consequently, raises the phase modulation frequency of the gyro to a level which is impractical for use with conventional modulators.
Conventional fiber optic rotation systems have restricted applications because of a persisting inability to maximize both sensitivity (by minimizing biasing phase noise) and linearity by achieving an adequate compromise between the abovediscussed conflicting constraints involved in matching the effective coil length to the phase modulation frequency. The present invention effectively and conveniently realizes such a compromise, as will be described below in detail.